In order to provide privacy in a communication system, apparatus is used that renders an analog communication signal unintelligible by altering or "scrambling" the signal in a prearranged way. The intended receiving party uses apparatus to descramble the signal and recover the transmitted information easily while any unintended receiving party experiences considerable difficulty in doing so. Such apparatus finds utility in the field of military, police or other official communications and in the field of civilian communications such as provided by the domestic telephone system. Throughout the following description, the analog communication signal is assumed to be speech, and the communication channel is assumed to be a telephone channel, although it will be understood that wider application of these techniques is envisioned and may include virtually any analog signal and any communication channel having limited bandwidth.
Speech scrambling is provided in the prior art in two basically dissimilar ways, analog scrambling and digital scrambling.
In one type of analog scrambling system, the speech signal is divided into one or more frequency subbands. Signals appearing in these subbands are inverted or the subbands are rearranged or otherwise scrambled in order to produce an unintelligible signal. Analog scrambling has the advantage of inband scrambling. That is, the scrambled signal is limited in bandwidth to the bandwidth of the original signal. Thus a 3.5 KHz telephone speech signal will occupy approximately 3.5 KHz in scrambled form and can be transmitted over ordinary telephone lines without the necessity for additional bandlimiting of the scrambled signal and the resulting unwanted distortion.
The disadvantage of analog scrambling is the limited security offered. Because of the complexity and precision required by the circuitry employed, the speech signal can be conveniently divided into relatively few frequency bands, and these may be interchanged in relatively few ways. A determined interceptor may find it straightforward to descramble the intercepted signal by exhaustively trying all possible combinations of the scrambling variables.
Digital scrambling has the potential for being more secure than analog scramblers. In digital scrambling, the speech signal is first encoded by an analog-to-digital converter into a convenient digital format. In one such format, eight-bit binary numbers are used to represent the speech waveform amplitude at repeated sample intervals. The binary digits of the sampled waveform are then subjected to digital scrambling. Existing techniques for digital encryption may be used to obtain virtually any desired degree of security.
The disadvantage of digital scrambling in a practical transmission system such as a telephone system is a substantial increase in bandwidth. A sampling rate of 8000 samples per second is suitable for a 3.5 KHz speech signal. With eight-bit samples, this results in a potential scrambled signal bit rate of 64 Kbps. For transmission over a telephone channel this will require a bandwidth considerably in excess of 3.5 KHz. Alternatively, techniques may be employed to reduce required bandwidth to 3.5 KHz, but these techniques introduce unwanted distortion and result in a loss of fidelity.
It has, therefore, been a problem in the prior art to provide a scrambling system that has the advantage of the high security afforded by digital scrambling without expanding bandwidth of the scrambled signal and thus either requiring a broadband communication channel or inducing distortion and loss of fidelity. Restated, the problem is to provide a secure inband digital speech scrambling system.